1. Field of the Invention The present invention relates to an x-ray tube for generating x-rays.
2. Related Background Art
X-rays are electromagnetic waves for which most of materials and objects exhibit favorable transmitting properties, and are often used for nondestructive/noncontact observation of internal structures of objects. Usually, an x-ray tube is used for generating x-rays, and electrons emitted from an electron gun are caused to collide against a target, so as to generate x-rays. In general, for this collision, the center axis of a tubular member accommodating the electron gun therein and the center axis of a tubular member accommodating the target therein are aligned with each other, or their center axes are set perpendicular to each other.
As such x-ray tubes, those described in U.S. Pat. (USP) Nos. 5,077,771 and 5,563,923 have been known. FIG. 12 is a configurational view of an x-ray tube in which the center axis of a tubular member accommodating an electron gun therein and the center axis of a tubular member accommodating a target therein are substantially orthogonal to each other, the x-ray tube being disclosed in U.S. Pat. No. 5,077,771. As shown in FIG. 12, this x-ray tube comprises an electron gun portion 910 for generating/emitting electrons; and an x-ray generating portion 920 for receiving the electrons emitted from the electron gun portion 910, in which the electrons collide against a target 921, so as to generate x-rays.
Here, the electron gun portion 910 comprises a heater 911 for generating heat in response to an electric power supplied thereto from the outside; a cathode 912 for emitting electrons when heated by the heater 911; a focus grid electrode 913 for accelerating/converging the electrons emitted from the cathode 912; and a container 914 which accommodates the heater 911, cathode 912, and focus grid electrode 913 therein and has an electron passage port.
The x-ray generating portion 920 comprises the target 921 against which electrons emitted from the electron gun portion 910 collide, so as to generate x-rays; a hood electrode 922 formed like a flat tube enveloping the target 921 with its center axis being substantially orthogonal to the center axis of the electron gun portion 910, while having an electron passage opening in a path through which the electrons emitted from the electron gun portion 910 reach the target 921; a container 923 having an inner space for accommodating the target 921 and hood electrode 922 therein and having an opening for taking out the x-rays generated at the target 921, the inner space being connected to the inner space of the container 914 by way of the electron passage port of the container 914; and an x-ray takeout window 924 made of an x-ray transmitting member and disposed at the x-ray passage port of the container 923. A positive high voltage is applied to the hood electrode 922 and target 921 with reference to the potential at the emitting port of the electron gun portion 910.
In the x-ray tube of FIG. 12, the electrons emitted from the electron gun portion 910 are accelerated to a high speed by the electric field between the focus grid electrode 913 and the hood electrode 922, so as to advance in a vertical direction (i.e., electric field direction) of an equipotential surface at each position of the electrons at a given time, thereby colliding against the target 921 after passing through the electron passage opening of the hood electrode. When the electrons collide against the target 921, x-rays are generated, and the x-rays are outputted from the x-ray tube by way of the x-ray passage opening of the hood electrode 922 and x-ray passage window 924 in succession.
For quality control of parts and the like, x-ray tubes are used as an x-ray source in x-ray inspection apparatus for yielding magnified penetration images and the like. Also, capability of increasing the magnification rate is quite important in improving the accuracy of inspection.
FIG. 13 is a typical configurational view of such an x-ray inspection apparatus. In the x-ray inspection apparatus shown in FIG. 13, x-rays emitted from an x-ray tube 107 irradiate a sample on a sample dish 105. The x-rays transmitted through the sample are detected by an x-ray/fluorescence multiplier (an image intensifier tube: I.I. tube) 102, and a magnified penetration image is picked up by an image pickup tube 101. The magnification rate of the penetration image in this apparatus is determined by the ratio between the distance (A) from the x-ray generating point (the focal position of the x-ray tube) 106 within the x-ray tube to the sample position and the distance (B) from the sample position to the x-ray entrance surface of the I.I. tube. That is, the magnification rate M is expressed by
M=(A+B)/A.xe2x80x83xe2x80x83(1)
Normally, A less than  less than B, and therefore the expression (1) can be represented by
M=B/A.xe2x80x83xe2x80x83(2)
Namely, for yielding a greater magnification rate, decreasing A or increasing B may be considered. Increasing B, however, not only enhances the overall size of the x-ray inspection apparatus, but also remarkably increases its weight by requiring a greater amount of lead shield 103 for keeping the x-rays from leaking outside, and so forth. Therefore, it is desirable that A be as small as possible.
Hence, in view of the foregoing, it is an object of the present invention to provide an x-ray tube which can shorten the distance from the x-ray generating point to the x-ray emitting window.
The present invention provides an x-ray tube comprising an electron gun for emitting an electron; a target for receiving the electron emitted from the electron gun at a front end face and generating an x-ray; an x-ray emitting window, disposed in front of the front end face of the target, for emitting the x-ray; and a hood electrode, formed as a tubular body attached to a tip portion of the target, having a peripheral face provided with an electron passage port for passing the electron therethrough, the electron passage port widening more on a side opposite from the x-ray emitting window than on the x-ray emitting window side with respect to a position intersecting an extension of the electron gun in an electron emitting direction. Also, the present invention provides an x-ray tube in which, of the tip portion of the target, a part exposed from the electron passage port is cutoff. Further, the present invention provides an x-ray tube in which the electron is made incident onto a center axis of the front end face of the target.
According to these aspects of the invention, the electron emitted from the electron gun pass through the electron passage port of the hood electrode and are made incident on the front end face of the target. Here, since the electron passage port is formed wider on the opposite side in the x-ray emitting direction, the electron is bent toward the x-ray emitting direction, so as to be made incident at a position near the x-ray emitting window. As a consequence, the distance between the x-ray generating position and the x-ray emitting window can be shortened.
Also, the present invention provides an x-ray tube comprising an electron gun for emitting an electron; a target for receiving the electron emitted from the electron gun In at a front end face and generating an x-ray; an x-ray emitting window, disposed in front of the front end face of the target, for emitting the x-ray; and a hood electrode formed as an annular body attached to a tip portion of the target, the hood electrode being disposed closer to the x-ray emitting window than is a position where the electron is made incident on the front end face.
According to this aspect of the invention, the electron emitted from the electron gun passes behind the hood electrode and is made incident on the front end face of the target. Here, since the electric field in the area where the electron passes is tilted toward the x-ray emitting window due to the existence of the hood electrode, the electron is bent toward the x-ray emitting direction, so as to be made incident at a position near the x-ray emitting window. As a consequence, the distance between the x-ray generating position and the x-ray emitting window can be shortened.
The present invention will be more fully understood from the detailed description given hereinbelow and the accompanying drawings, which are given byway of illustration only and are not to be considered as limiting the present invention.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will be apparent to those skilled in the art from this detailed description.